docs: def clean(self):
          """ Validates the considered instance. """
          super().clean()
  
          # At least a poster (user) or a session key must be associated with
          # the vote instance.
          if self.voter is None and self.anonymous_key is None:
              raise ValidationError(_('A user id or an anonymous key must be used.'))
          if self.voter and self.anonymous_key:
              raise ValidationError(_('A user id or an anonymous key must be used, but not both.'))